Lecture 13: Eusociality

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APS209 Animal Behaviour

Social Insects - Eusociality

Aims

1. To define eusociality and describe its distribution among insects

2. To explain the role of ecological and genetic predispositions in the evolution of eusociality, with particular reference to haplodiploidy
3. To examine sources and outcome of conflicts over reproduction within social insect colonies

 

Objectives

1. To understand the likely evolutionary routes to eusociality
2. To understand the consequences of haplodiploidy for genetic relatedness within colonies
3. To understand potential sources of conflict among queens and workers in social insects

Eusociality – Eusociality describes social systems where there is cooperative brood care, generational overlap, and, critically, sterile castes. Eusociality has evolved independently many times in the Hymenoptera (ants – 9500 spp, bees – 1000 spp. and wasps – 800 spp), and also in Isoptera (termites – 2000 spp.) and Homoptera (aphids). Eusocial species have a dominant ecological presence in most parts of the world, and have evolved remarkable specialisations associated with their social way of life.

Evolution of eusociality – There are two hypotheses for the evolution of eusociality from solitary ancestral forms

     Staying at home: from a solitary parasitoid that guards its nest against parasites, it is proposed that young stayed at home to help their mother defend and build the protective nest. As nests become more elaborate, the benefit of staying and helping raise siblings could outweigh the benefit of independent breeding. Remember that full siblings are as closely related as offspring in diploid organisms, and from the mother’s perspective, offspring are more closely related than grand-offspring.

     Sharing a nest: in many wasps, nests are founded by a group of cooperating females, very often sisters. In primitively social wasps, all females reproduce, but in others one ‘queen’ dominates reproduction, setting the scene for the evolution of workers. The ecological factors leading to shared nesting are probably similar to those described above: predation/parasitism and nest-building costs. Again, relatedness will ensure that non-reproductive females would benefit from the queen’s reproduction.

Contemporary subsocial halictine bees follow the ‘stay-at-home’ model, while parasocial Polistes and stenogastrine wasps follow the ‘share-a-nest’ model.

E.g. Naked mole rats
1 dominant queens and many subservient non-reproductive helpers. When queen dies, females fight over succession.

Haplodiploidy
The Hymenoptera exhibit haplodiploidy: males develop from unfertilized eggs and are haploid; females develop from normally fertilized eggs and are diploid. Males form gametes without meiosis, so all his sperm are identical, while females form gametes through meiosis, as is usual in sexually reproducing organisms. This has important consequences for relatedness among individuals in the colony.

To calculate relatedness, draw a pedigree linking two individuals through their recent common ancestors. Draw arrows along the pathways and indicate alongside each pathway the probability that a copy of a gene will be shared. Remember that two individuals may differ in their perspective on mutual relatedness. See lecture slides on MOLE for other examples.

Relatedness among close relatives in a haplodiploid species.

               mother             father               sister                brother             daughter          son
Female     0.5                   0.5                   0.75                 0.25                 0.5                   0.5
Male         1                      0                      0.5                   0.5                   1                      0

In hymenoptera, workers are always female. In diploid termites they may be either sex. In eusocial aphids, colony members are clonal (genetically identical). Haplodiploidy results in interesting conflicts among colony members over reproductive options.

Conflicts over sex ratio
Queens are equally related to sons and daughters and so should invest equally in reproductive sons and daughters (1:1 ratio of investment). Workers are more closely related to sisters (0.75) than brothers (0.25) and so prefer to invest more in sisters (3:1 ratio of investment). Workers are more closely related to fellow workers and future queens than to own offspring.

A comparison across 21 ant species by Trivers and Hare (1976, Science 191: 249-263) suggested that workers win this conflict. But, preferred sex ratio of each party will also be influenced by:

(a)  Local Mate Competition - if sons compete with each other for matings it should pay to produce fewer of them, e.g. fig wasps

(b)  Queen mating frequency – if a female mates with multiple males, workers will be less closely related to each other, on average.


References

See Chapter 13 in Alcock’s Animal Behavior (2009). See also Chapter 13 in J.R. Krebs & N.B. Davies Introduction to Behavioural Ecology, 3rd edition (1993).

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